CFRP Strengthened RC Beams Research Articles

IRT research on influence of long-term loads on defects in FRP strengthened RC beams

It has been more than two decades, since FRP strengthening method was first time used in Poland. Therefore there is a natural need to develop an efficient quality assessment technique to verify design assumptions of strengthening in existing structures after many years. One of the promising non-destructive method of quality assessment is infrared thermography (IRT). In this paper, an initial study on recognition of delamination mainly in CFRP laminates using IRT was conducted as well as the influence of long-term loads on defects in CFRP strengthened RC beams was presented.

Effect of Preload Level on Flexural Load-carrying Capacity of RC Beams Strengthened by Externally Bonded FRP Sheets

Most of the laboratory tests investigated the flexural performance of un-preloaded or undamaged RC beams strengthened with CFRP composites. However, in engineering applications, the structural member must carry a certain load or damage. There is a lack of systematical investigations on the effects of preload or damage level on the flexural load-carrying capacity of CFRP-strengthened RC beams. This paper tested 22 RC beams to investigate the influence of preload level on flexural load-carrying capacity of CFRP-strengthened RC beams. The test variables are preload level, amount of CFRP sheets, tension rebar ratio, and concrete strength. The test results show that if the preload level is not more than 80% of the yielding strength of the original beam, the preload or damage level does not influence the flexural load-carrying capacity of CFRP-strengthened RC beams. However, the ultimate flexural load-carrying capacity is significantly poor than that of RC beam strengthened under a preload level not more than 80% of the yielding strength, if the RC beams are strengthened under a preload level more than 90% of the yielding strength.

Experimental study of RC jacketed and CFRP strengthened RC beams

This paper presents the results of an experimental study on retrofitting RC beams using FRP strengthening and RC jacketing. Three-point loadings are applied to twelve strengthened beams and three reference specimens to put the two techniques into perspective. Formation of first cracks, FRP debonding, and onset of concrete crushing are compared and discussed. Load–deflection behavior, failure modes, and crack propagation patterns are studied extensively. The experiments demonstrate that the RC jacketed beams exhibit a flexural failure identical to the reference beams. However, the FRP strengthened beams fail in a relatively small deflection due to the premature FRP debonding. Furthermore, the both techniques noticeably enhance strength and energy dissipation capacity. The applied RC jacketing can also improve ductility; however, the FRP method cannot provide sufficient ductility. Therefore, the RC jacketing can be used in seismic zones as an alternative to the FRP method. In addition, conventional analytical methods are applied for evaluation of ultimate moment, and the results are compared with the experiments. Good agreement between the results verifies the analyses.

Contribution to the understanding of the mechanical behaviour of CFRP-strengthened RC beams subjected to fire: Experimental and numerical assessment

Recent experimental tests and numerical simulations about the fire resistance behaviour of CFRP-strengthened RC beams proved that CFRP strengthening systems are able to attain considerable fire endurance, provided that adequate fire protection systems are used. In a fire event, even though a CFRP laminate may rapidly debond from the central part of the beam in which it is installed, if sufficiently thick insulation is applied in the anchorage zones, the laminate transforms into a “cable” fixed at the extremities, thus maintaining a considerable contribution to the mechanical response of the strengthened beam. This paper presents experimental and numerical investigations on CFRP-strengthened RC beams with the objective of understanding in further depth their fire resistance behaviour, namely the influence of the above mentioned “cable” mechanism on the mechanical response of the beams. The experimental campaign, performed at ambient temperature, comprised 4-point bending tests on RC beams strengthened with CFRP laminates according to either the EBR or the NSM techniques, in both cases fully or partially (only at the anchorages, thus simulating the cable mechanism) bonded to the soffit of the beams. For the test conditions used in this study, for both types of strengthening systems, partially bonding the CFRP laminates did not affect the stiffness of the beams and caused only a slight reduction of their strength (6–15%). The numerical study comprised the simulation of the structural response of all beams tested. Non-linear finite element models were developed in Atena commercial package, in which a smeared cracked model was adopted to simulate concrete and appropriate bond-slip constitutive relations were defined for the CFRP-concrete interfaces. A very good agreement was obtained between experimental data and numerical results, providing further validation to the “cable” mechanism and the possibility of taking it into account when designing fire protection systems for CFRP-strengthened RC beams.

Debonding Behaviors of CFRP Strengthened RC Beams with Weak Interfaces

Debonding behaviors of CFRP strengthened RC beams were experimentally investigated under the influence of weak interfaces, which are induced either by defective bonding of replaced cover or expansive cracks. Shown by test results, weak interfaces impaired considerably the structural integrity of strengthening systems during loading, and easily led to CFRP debonding failure. U-strips worked effectively in preventing the integral debonding and guarantee the structural performance of flexural sheets. However, local cover delamination in the loading process and premature rupture of flexural CFRP could still take place due to the weak interface effects. Therefore, allowable tensile strain of flexural CFRP should be reduced, and more strict confinement and anchorage measures should be taken in this case.

A Numerical Evaluation of Insulated CFRP-Strengthened RC Beams Exposed to Fire

There are mainly two approaches to improve the fire resistance of FRP systems. While the most common way is to protect or insulate the FRP systems, the other way is to use fibers and resins with better fire-performance. In this paper a numerical investigation for evaluating the fire behavior of insulated CFRP-strengthened RC beams is presented.The effects of external loading and thermal expansion of materials in both the structural and the thermal behavior of composite elements due to loading and elevated temperatures are taken into consideration in a finite element model. The validity of the numerical model isdemonstrated withthe results from an existing experimental study on insulated CFRP-strengthened RC beam. The conclusions of this investigation have been employed to predict the structural behavior of concrete structures successfully.

Assessment of CFRP strengthened RC beams through dynamic tests

The use of Fiber Reinforced Polymers (FRPs) for strengthening damaged RC beams has become common practice over recent years. Two methods adopted for repairing or strengthening such beams are FRP plates/sheets glued onto their concrete surface and FRP rods or strips inserted into grooves.This paper investigates experimental vibration monitoring of strengthening according to the two aforementioned methods through dynamic tests on six RC beam models strengthened using carbon FRP. Three beams were strengthened applying CFRP sheets on the tensile cracked surface after loading and three beams were strengthened by near surface mounted (NSM) CFRP rods. The experimental results include both the static tests to create damage and the dynamic tests of strengthened, measuring natural vibration modes and frequency values for free end beams.Comparison between experimental dynamic response and static behavior established that vibration monitoring is a convenient, non-destructive method for assessing strengthened beams under service loads. Further studies and tests must be developed in order to solve the issues that emerged following the analysis of the experimental data obtained.

Deflection and cracking behavior of RC beams externally reinforced with carbon fiber laminates

This article presents the results and discussions of experimental and analytical work on CFRP-strengthened RC beams investigating the cracking and deflection performance. The aim of this study is to evaluate the time-dependent behavior by different degrees of strengthening scheme and two types of sustained loads. Eight RC beams were cast and tested under sustained loads for cracked and uncracked section. An analytical method based on EMM is developed to predict the time-dependent behavior of CFRP externally bonded RC beams. A simplified tension stiffening model for calculating the time-dependent deflection is modified for CFRP-reinforced concrete beams.

Performance Evaluation of RC Beams Strengthened with an Externally Bonded FRP System under Simulated Vehicle Loads

Carbon fiber-reinforced polymer (CFRP) sheets can be used to strengthen existing RC bridge girders/decks. The objective of this study is to investigate whether transient vehicle loads present during the installation of CFRP influence the bond performance between the CFRP and the concrete substrate and, therefore, the consequent strengthening effect. A total of eight RC beams were tested. Two were unstrengthened reference beams, five were strengthened with CFRP sheets while subject to transient loads, and the final beam was strengthened with CFRP sheets while subject to only a sustained static load. The test parameters included the amplitude of the transient load, the anchorage length of the CFRP sheets, and the reinforcing/strengthening ratio. The transient loads were continued for 2 days (during CFRP cure) before all five CFRP-strengthened RC beams were tested to failure in four-point flexure. The test results were compared with those of the reference beams and the one strengthened under a sustained load. It was shown that a 1-Hz sinusoidal transient load varying between 30 and 50% of the ultimate capacity of the unstrengthened beam during the installation and curing of the CFRP sheets does not affect the structural performance of CFRP-strengthened RC beams. This result demonstrates the applicability of the fiber-reinforced polymer (FRP) strengthening technique for bridge girders that are subjected to continuous vehicle loads.

Experimental study on long-term behaviour of CFRP strengthened RC beams under sustained load

The strengthening and rehabilitation of reinforced concrete structures with externally bonded carbon fibre reinforced polymer (FRP) laminates has shown excellent performance and, as a result, this technology is rapidly replacing steel plate bonding techniques. This paper addresses this issue, and presents results deals with the influence of external bonded CFRP-reinforcement on the time-dependent behavior of reinforced concrete beams. A total of eight reinforced concrete beams with cracked and uncracked section, with and without externally bonded CFRP laminates, were investigated for their creep and shrinkage behavior. All the beams considered in this paper were simply supported and subjected to a uniform sustained loading for the period of six months. The main parameters of this study are two types of sustained load and different degrees of strengthening scheme for both cracked and un-cracked sections of beams. Both analytical and experimental work has been carried out on strengthened beams to investigate the cracking and deflection performance. The applied sustained load was 56% and 38% of the ultimate static capacities of the un-strengthened beams for cracked and un-cracked section respectively. The analytical values based on effective modulus method (EMM) are compared to the experimental results and it is found that the analytical values are in general give conservative estimates of the experimental results. It was concluded that the attachment of CFRP composite laminates has a positive influence on the long term performance of strengthened beams.

Deflection Prediction of FRP-Strengthened Reinforced Concrete Beams

Bonding fiber reinforced polymer (FRP) laminates to the tension face of RC members has been proven to be an effective method to improve the flexural strength. However, structural members are not only needed to have adequate strength, but also to have adequate performance of deformation at service load levels. To evaluate the deflection of externally FRP-strengthened RC beams, a total of 18 RC beams, including 16 beams strengthened with CFRP laminate under different preload levels and 2 control beams, were tested. Based on the assumption that the section of the beam behaves a tri-linear moment-curvature response characterized by pre-crack stage, post-crack stage and failure stage and the test results, this paper presents a modified model to evaluate the deflection of CFRP-strengthened RC beams. The present modified model was verified by the similar test results, and shows a good agreement with the test results.

Experimental Investigation on Effective Bond Length of Flexurally CFRP-Strengthened RC Beams Failed by Intermediate Crack Debonding

The models to calculate the effective bond length determined by single-lap or double-lap shear tests are more suitable for the end-debonding failure; and the improved models based on the simply shear test models to calculate the effective bond length of flexural CFRP-strengthened beams failed by intermediate crack debonding (IC debonding) are also lack of the support of experimental data. To investigate the effective bond length of flexural CFRP-strengthened beams, a total of 9 flexurally CFRP-strengthened beams were tested. The experimental results show that the existing models are very conservative for calculating the effective bond length in flexural FRP-strengthened RC beams failed by IC debonding. Based on the experimental results, this paper presents a model to calculate the effective bond length of flexural FRP-strengthened RC beams failed by IC debonding, and a model to calculate the plastic hinge length at the shear-flexure span.

Fatigue Performance of CFRP Strengthened RC Beams under Environmental Conditioning and Sustained Load

Carbon fiber-reinforced polymers CFRPs have become increasingly important in recent years in bridge rehabilitation. Significant research has been done on the static behavior of CFRP-strengthened reinforced concrete RC structures; however, the fatigue behavior of such structures with interface defects subjected to harsh environmental conditions still needs to be investigated. Hence, an experimental program has been carried out to investigate the fatigue behavior, under a load range, which generates service load stress levels, of RC beams strengthened with CFRP fabrics. The effect of aggressive environments was studied by subjecting the test members to freeze-thaw, extreme temperature, ultraviolet light exposure, and relative humidity cycles. All beams survived 2 million fatigue cycles without showing significant bond degradation between composite and substrate. However, significant flexural stiffness degradation was observed in the conditioned specimens. The presence of defects also affected specimen stiffness; however, limited growth in defect size was observed due to fatigue cycling.

Fiber Optic Sensors for Strain Measurement of CFRP-strengthened RC Beams

There is a growing need for built-in monitoring systems for new and aging civil engineering structures, due to problems such as increasing traffic loads and rising costs of maintenance and repair. Fiber optic sensors FOS), capable of reading strains, loads, deflections, and temperature are promising candidates for life-long health monitoring of these structures. However, since FOS have only been introduced recently into the field of structural monitoring, their acceptance and widespread implementation will be conditioned by their durability under severe climatic and loading conditions. This article reports on the performance of strain extrinsic FOS attached to carbon fiber-reinforced polymer CFRP) plates used to strengthen concrete structures. The specimens tested in this project are reinforced concrete RC) beams with an additional external CFRP reinforcement. The strain data obtained from the FOS were compared with data obtained from collocated electrical strain gauges. The FOS-instrumented beams were first subjected to fatigue loading for various numbers of cycles and load amplitudes. Then they were tested monotonically for failure under four-point-bending. The test results provide an insight on the fatigue and postfatigue behavior of FOS used for strain measurement in reinforced concrete structures.

CFRP-Strengthened and Corroded RC Beams under Monotonic and Fatigue Loads

An experimental program has been carried out to investigate the structural behavior of RC beams strengthened by carbon-fiber–reinforced polymer (CFRP) sheets and exposed to a corrosive environment. A total of eight specimens (120 × 175 × 2,000 mm) were tested. Six specimens were CFRP strengthened and corroded, one specimen was unstrengthened and corroded, and one specimen was neither strengthened nor corroded. Two different strengthening schemes were applied: (1) wrapping the specimen with CFRP sheets; and (2) both specimen wrapping and flexural strengthening. Three specimens were tested under monotonic loading and five specimens were tested in fatigue. The results showed that the use of CFRP sheets for strengthening RC beams that are experiencing steel reinforcement corrosion is an efficient technique that can maintain the structural integrity and enhance the structural behavior of such beams. The ultimate monotonic strength of the CFRP strengthened-corroded specimens increased to a level between 37 and 87% above the predicted strength of a similar unstrengthened-uncorroded (virgin) specimen. The fatigue life of the CFRP strengthened-corroded specimens was increased within a range of 2.5–6.0 times that of a similar unstrengthened-corroded specimen but was lower than that of the uncorroded (virgin) specimen.